The present invention relates to a method of sizing rolling a wire by a 4-roll rolling mill.
A conventional example of a sizing rolling method using a 4-roll rolling mill (rolling method of manufacturing a bar and wire having a different diameter using the same rolling rolls and changing a gap therebetween) is disclosed in, for example, Japanese Examined Patent Application Publication No. 3-6481.
In this method, two sets of 4-roll rolling mills, which incline a rolling direction by 45xc2x0 each other, are installed in series along a rolling line. Each of the rolling rolls used here includes a groove formed on the outer circumferential surface thereof and the groove is composed of an arc-shaped perfect circle forming section and a suitable escape section. Further, the publication describes it is preferable that the central angle of the perfect circle forming section of the groove be as small as possible and that an amount of escape (the angle between the escape section and the perfect circle forming section) be as large as possible in the range in which no flaw is caused in order to increase a size range (hereinafter, referred to as xe2x80x9csize free rangexe2x80x9d) of products which can be sizing rolled by the same rolls within an allowable range of dimensional error.
In contrast, the applicant proposed a sizing rolling mill capable of increasing the size free range in consideration of bitten-out and falling-down which are a problem in actual rolling carried out by 4 rolls while suppressing the occurrence of the problem (refer to Japanese Unexamined Patent Application Publication No. 9-225502). In the proposal, at least three sets of 4-roll rolling mills are installed and the central angles of the perfect circle forming sections of the grooves of the respective rolling rolls are set to at least 15xc2x0 in a first path, to at least 45xc2x0 in a final path and to at least 30xc2x0 in an intermediate path (second path).
xe2x80x9cBitten-outxe2x80x9d used here means that a workpiece 7 is protruded form the space (caliber) surrounded by the outer circumferential surfaces of 4 rolling rolls 50 as shown in FIG. 5. When the center of the workpiece 7 does not pass through the center of the caliber in rolling, the bitten-out is liable to be arisen. When an amount of bitten-out is large, a bitten-out portion 75 is folded in the caliber in the next path and made to a folded flaw.
Further, xe2x80x9cfalling-downxe2x80x9d means that the workpiece 7 is rotated between paths as shown in FIG. 6. A degree of falling-down is represented by the angle (falling-down angle) xcex3 between the line L1 showing the central position of one of the rolling rolls 50 in the width direction thereof and the line L2 showing the position of the workpiece 7 which is to be located at the central position of the rolling roll 50 in the width direction thereof in the path. When an amount of falling-down is large, a faulty product is made due to a surface flaw and a bent flaw.
In the sizing rolling mill disclosed in Japanese Unexamined Patent Application Publication No. 9-225502, however, it has been found that there is a tendency that is liable to be arisen in a second path particularly in a wire having a small diameter (for example, a wire having a diameter of 7 mm or less) and there is a possibility that a bent flaw is caused in a third path accordingly.
An object of the present invention is to obtain an excellent surface property in a sizing rolling method which is carried out by at least 3 sets of 4-roll rolling mills installed in series in a wire particularly having a small diameter.
To solve the above problem, an invention according to claim 1 provides a wire sizing rolling method which is characterized by comprising the steps of installing at least 3 sets of 4-roll rolling mills in series with each rolling mill having 2 pairs of 4 rolling rolls each having a groove formed on an outer circumferential surface and composed of an arc-shaped perfect circle forming section and escape sections and setting central angles of the perfect circle forming sections of the rolling rolls to less than 15xc2x0 in the first path of final three paths, to at least 30xc2x0 in the second path thereof, and to at least 45xc2x0 in the third path thereof; and installing a roller guide on the inlet side of the second path and introducing a workpiece to the second path while holding and guiding the workpiece by the guide rollers of the roller guide such that the surface of the workpiece which was not rolled in the first path is located at the central portions of the guide rollers.
According to this method, since the central angle of the perfect circle forming section of each rolling roll in the first path of the final three paths is set to less than 15xc2x0 as well as the free surface of the workpiece is held and guided by the guide rollers of the roller guide installed on the inlet side of the second path, the holding/guiding capability of the guide rollers of the roller guide for the workpiece can be improved as compared with the case in which the central angle of the perfect circle forming section of each rolling roll in the first path is set to at least 15xc2x0, whereby it is made difficult for falling-down to occur in the second path. Further, in addition to the above, since the central angle of the perfect circle forming section of each rolling roll is set to at least 30xc2x0 in the second path of the final three paths and to at least 45 in the final path, an excellent surface property can be obtained in a resulting product.
An invention of claim 2 is characterized in the wire sizing rolling method according to claim 1 such that the escape section of each rolling roll in the first path is formed by straight lines, the straight lines act as tangential lines to both the ends of the arc forming the perfect circle forming section, a V-shaped groove for holding and guiding the workpiece is formed on the outer circumferential surface of each guide roller of the roller guide installed on the inlet side of the second path, and an angle of the V-shaped groove is set similar to the angle between the straight lines forming an escape section between the adjacent rolling rolls in the first path.
An invention according to claim 3 provides a wire sizing rolling method of sizing rolling a wire by a 4-roll rolling mill each having two pairs of 4 rolling rolls, the method being characterized by comprising the steps of installing at least 3 sets of the 4-roll rolling mills in series, using flat rolls as rolling rolls each having an outer circumferential surface on which no groove is formed in the first path of final three paths, and using rolls as rolling rolls each having an outer circumferential surface having a groove composed of an arc-shaped perfect circle forming section and escape sections in the second and subsequent paths thereof with the central angles of the perfect circle forming sections of the respective rolling rolls set to at least 30xc2x0 in the second path and to at least 45xc2x0 in the third path; and installing a roller guide on the inlet side of the second path and introducing a workpiece to the second path while holding and guiding the workpiece by the guide rollers of the roller guide such that the surface of the workpiece which was not rolled in the first path is located at the central portions of the guide rollers.
In the 4-roll rolling mill, since one rolling roll receives the cross section of the workpiece in the range of 90xc2x0, when an arc-shaped groove is formed on the outer circumferential surface of each rolling roll, the central angle of the arc is 90xc2x0 or less. Then, when the central angle of the arc is 0xc2x0, the rolling roll is arranged as the flat roll whose outer circumferential surface has no groove. Accordingly, the method of claim 3 corresponds to the case in which the central angle (xcex81) of the perfect circle forming section of each rolling roll in the first path is set to 0xc2x0.
That is, in the method of claim 1, since it is set forth as a premise that the grove of each rolling roll is composed of a perfect circle forming section and escape sections also in the first path, it is interpreted that the method, in which the flat roll of xcex81=0 is used as each rolling roll in the first path, is not included in the method of claim 1. However, actually, an effect of making the occurrence of falling-down difficult is higher in the case of xcex81=0 than the case of 0 less than xcex81 less than 15xc2x0, claim 3 is provided independently of claim 1.
An invention of claim 4 is characterized in the wire sizing rolling method according to claim 3 such that a V-shaped groove for holding and guiding the workpiece is formed on the outer circumferential surface of each guide roller of the roller guide installed on the inlet side of the second path, and an angle of the V-shaped groove is set to 90xc2x0.